1. Field of the Invention
The present invention relates to a laser module and a production process thereof, and particularly to a laser module in which structural members including a semiconductor laser with an oscillation wavelength of 350 to 450 nm are hermetically sealed, and a production process thereof.
Further, the present invention relates to a laser module in which a semiconductor laser element is disposed in a hermetic container.
Further yet, the present invention relates to a laser device which multiplexes laser beams emitted from a plurality of semiconductor laser elements to an optical fiber by use of a condensing optical system.
2. Description of the Related Art
Heretofore, in optical modules which irradiate or emit ultraviolet rays with a wavelength of 400 nm or less, optical losses of optical components included in optical modules have increased due to the irradiated or emitted ultraviolet rays, and there has been a problem with characteristics of the optical components deteriorating. Such optical losses are thought to occur because moisture, oils and the like in the atmosphere are broken down by the ultraviolet rays, and decomposed components thereof are deposited on surfaces of the optical components.
Accordingly, in an ultraviolet ray-irradiating optical system described in Japanese Patent Application Laid-Open (JP-A) No. 11-167132 and the like, an atmosphere in which optical components are disposed (a sealed atmosphere) is nitrogen with a high purity of 99.9% or more, dry air with a high purity of 99.9% or more, air with a moisture content of not more than 0.1%, air with 0.1% or less of hydrocarbon compounds, or the like. Thus, the deposition of decomposition components is avoided, and a reduction in the output of ultraviolet laser light is prevented.
When sealed atmospheres of modules containing semiconductor lasers with oscillation wavelengths of 350 to 450 nm have been analyzed, numerous compounds have been found to be included in the sealed atmospheres. Among these compounds, particular organic gas components, which are generated from solid organic materials and adhere to optical members, mechanical members and the like that are employed in the module, are principally responsible for deterioration of laser characteristics.
In conventional laser modules, laser elements and optical systems are fixed. For this purpose, organic adhesives and the like are employed, such as, for example, epoxy-based adhesives such as an adhesive disclosed in JP-A No. 2001-177166, NOA61 manufactured by NORLAND and the like. In addition, many of the solid organic materials that adhere to optical members, mechanical members and the like used in the module are mixed in with the sealed atmosphere in a module fabrication process. Thus, even if scrubbing is implemented, organic residues occur. Organic gas is generated from the solid organic materials, and the generated gas (known as “outgas”) fills the sealed laser module in a certain amount. Depending on the types of the solid organic materials, compounds including silicon atoms, phosphorus atoms, sulfur atoms and the like may be contained in this outgas.
Further still, in a sealing process, small molecules and low boiling point organic gas components of solvents and the like which are employed for washing components that structure interior portions of the module are mixed in with the sealed gas, and act as impurities. Such solvents include isopropyl alcohol (molecular weight 60.10, boiling point 82.4° C.) and acetone (molecular weight 58.08, boiling point 56.1 to 56.5° C.). The sealed gas may be dry nitrogen, dry air or the like.
Consequently, the organic gas components that are generated from the solid organic materials (below referred to as “outgas components”) and the organic gas components that are mixed in during the sealing process (below referred to as “impurity components”) are present in the sealed atmosphere. When these respective components are analyzed by gas chromatography, distributions of molecular weights and boiling points of the respective components are clearly different, as shown in FIGS. 5 and 6.
FIG. 5 is a distribution with respect to molecular weight for the impurity components and the outgas components respectively, with index amounts of components detected by a GC-MASS (a gas chromatograph mass spectrometer) being set to 100%. FIG. 6 is a distribution of boiling points of the components detected by the GC-MASS shown as a distribution of boiling point proportions, with total amounts for the impurity components and the outgas components being respectively set to 100%.
Thus, the molecular weights of the outgas components are distributed in a range of 70 and greater and, the molecular weights of the impurity components are distributed in a range below 70. While the boiling points of the outgas components are distributed in a range of 70° C. and upward, the boiling points of the impurity components are distributed in a range of less than 100° C.
Next, a relationship between density of the aforementioned two types of organic gas in the sealed atmosphere and a rate of deterioration of the module will be described for a laser module. This laser module has the same structure as a laser module shown in FIGS. 1 to 4, which is described later, except with regard to the employment of organic adhesive. Results are shown in FIG. 7. The rate of deterioration of the module is expressed by an amount of increase, per one hour of driving, in a current that is required for driving all elements in a case in which each light emission point of the laser module is driven at 100 mW.
The plot of black diamonds shows a relationship between density of outgas components and the rate of deterioration of the module, and the plot of white squares shows a relationship between density of impurity components and the rate of deterioration of the module. The density of the impurity components was varied by manual adjustment of a density of acetone in the sealed gas.
As can be seen from FIG. 7, the rate of increase in the driving current rises sharply when the density of the outgas components in the sealed atmosphere goes above 1000 ppm, and the deterioration of the module accelerates remarkably. It is assumed that a cause of this rapid acceleration is solid components, which are generated by optical decomposition of the outgas components, being deposited on surfaces of light-emitting portions, optical components and the like that are contained in the module.
However, even when the density of the impurity components goes above 1000 ppm, deposition of solid components on the light-emitting portions and optical components contained in the module is not observed. This is because, even when the impurity components are optically decomposed, the decomposed materials are not solid at usual temperatures, and thus will not be deposited. Even when the acetone impurity component was substituted with isopropyl alcohol, deposition of solid components was not observed, the same as with acetone.
Now, laser modules in which a main laser device, a collimator lens, a condensing lens, an optical fiber and the like are sealed in a hermetic container are also known. In these laser modules, contaminant materials that remain in the sealed container adhere to optical components, such as an emission end face of a semiconductor laser element, the lenses and the optical fiber, and the like, and there is a problem with laser characteristics deteriorating. The contaminant materials include hydrocarbon compounds that are mixed in with an atmosphere during a fabrication process. It is known that these hydrocarbon compounds are polymerized or decomposed by laser light, and then adhere.
Various methods have been proposed to solve this problem, as shown below. For example, JP-A number 11-167132 discloses that, in order to prevent a reduction in output of laser light at 400 nm or less, it is effective to set the amount of hydrocarbon compounds in the container to be 0.1% or less, and deposition of optically decomposed components of the hydrocarbon compounds on the optical components and the like can be consequently prevented. Further, it has been proposed that the sealed atmosphere be dry air. Hence, an effect of deposited matter being eliminated by photochemical reactions of oxygen in the atmosphere with the deposited hydrocarbon compounds can be expected.
Further, U.S. Pat. No. 5,392,305 discloses, in order to prevent the adherence of hydrocarbon compounds on semiconductor laser end faces due to optical decomposition of hydrocarbon-based gases, mixing at least 100 ppm of oxygen in with the sealed gas, for the purpose of decomposing the hydrocarbon-based gases.
Further still, JP-A No. 11-87814 discloses that it is possible to assure long-term reliability by degreasing and scrubbing, to remove contaminant materials such as oils and the like.
Patent Application No. 2000-336850, from the present applicant, proposes a laser module which employs a GaN-type semiconductor laser element having an oscillation wavelength of 350 to 450 nm. However, because the energy of short wavelength laser light is high, hydrocarbon-based gases that are present in the module are polymerized or decomposed, and a rate of adherence to an end face of the semiconductor laser element, optical components and the like is high, which is particularly problematic.
Hydrocarbon-based deposits that are generated by reactions between laser light and hydrocarbon compounds are, as illustrated in the aforementioned U.S. Pat. No. 5,392,305, eliminated by being decomposed to CO2 and H2O with a gas atmosphere which includes oxygen in at least a certain amount.
However, such deposits are not just hydrocarbon compounds; the presence of silicon compounds has also been confirmed. Thus, it has been found that these deposits cannot be decomposed and eliminated merely by including oxygen in the atmosphere. Deposited silicon compounds are generated by photochemical reactions between laser light and organic compound gases that include silicon (below referred to as organic silicon compounds), such as siloxane combinations (Si—O—Si), silanol groups (—Si—OH) and the like. In addition, the presence of oxygen in the atmosphere has the effect of increasing reaction rates of these reactions. Because the deposits of hydrocarbon compounds and silicon compounds are generated by optical absorption, there is a problem in that reliability over time is greatly degraded by continuous oscillation.
Silicon compounds as referred to herein represent any silicon compounds having structures which include silicon, whether organic or inorganic, and include inorganic silicon oxides and organic silicon compounds.
A principal source of generation is gases that are generated from silicone members which are employed at arbitrary locations in a laser module production process. These gases may adhere to surfaces of each component inside the laser module. Furthermore, small amounts of these gases are included in the sealed gas when the module is sealed and employed. As methods for eliminating gas components in these processes, operation in a usual clean room, disposition in a sealed gas purification device and the like are available. However, even with these methods, such gases cannot be completely eliminated, and large investments in equipment are required. Even if a degreasing process is undergone as described in JP-A No. 11-87814, it is impossible to completely avoid mixing in of the aforementioned compounds from the atmosphere of the production process.
It is possible to remove the gases of hydrocarbon compounds and silicon compounds that adhere to the components in the laser module by decomposing and vaporizing the gases with a heat treatment at a temperature of 200° C. or more, preferably at least 300° C. However, this heat treatment requires a treatment duration of several hours or several tens of hours, and if the components in the module are fixed with an organic adhesive, mechanical characteristics of the adhesive will deteriorate due to thermal degradation. Therefore, this method cannot be employed.
Also, as devices which emit ultraviolet region laser beams, a multiplex laser light source which is capable of raising output and an exposure light source which is formed of a plurality of such multiplex laser light sources have been proposed. This multiplex laser light source is equipped with a plurality of semiconductor laser elements in a hermetic container, a single multimode optical fiber, and a condensing optical system which focuses laser beams emitted from the plurality of semiconductor laser elements on the multimode optical fiber.
In the multiplex laser light source described above, organic gases, which are generated from small particles present in the container and from adhesive of the respective members and the like, and/or hydrocarbon compounds, which are mixed in during fabrication, adhere to emission end faces of the semiconductor laser elements, lenses, an incidence end face of the optical fiber and the like. This causes a problem with deterioration of laser characteristics. It is known that the hydrocarbon compounds are particularly subject to polymerization and/or decomposition by laser light of short wavelengths, and that increases in output are disrupted by adherence of the polymerized and/or decomposed materials. Furthermore, because the container is hermetically sealed, the density of organic gases in the container is increased by increases in temperature and the like, and there is a problem that adhering amounts are further increased.
Furthermore, it has been found that low molecular weight siloxanes that are suspended in air react with oxygen in a photochemical reaction due to the ultra-violet rays, and are deposited and adhered at an optical glass window component in the form of silicon oxides. Accordingly, periodic replacement of a “window” member that contacts the external atmosphere is recommended.
In order to solve such problems, it has been proposed to mix at least 100 ppm of oxygen in with the sealed gas, for the purpose of decomposing hydrocarbon compounds to carbon dioxide and water.
Further, for an optical system which irradiates ultraviolet light of 400 nm or less at optical components, it has been proposed to make the atmosphere of the optical system at least 99.9% nitrogen (by volume).
Further still, degreasing and scrubbing of oil contents and the like in laser devices has been proposed.
In the case of the exposure light source using a plurality of multiplex laser light sources as described above, adjustment of the atmosphere at each container causes an increase in processing and/or fabrication inconsistencies, and leads to an increase in costs.
Further yet, at the hermetically sealed container, in order to improve the hermiticity, KOVAR™ (an alloy of iron, nickel and cobalt), which has substantially the same coefficient of thermal expansion as the container material, is employed as an adhesive for attaching electrode terminals to the container. However, because kovar is expensive, in the case of an exposure light source that uses a plurality of multiplexed laser light sources, there is a problem that costs rise in accordance with the increase in the number of multiplex laser light sources.
Now, Patent Application No. 2002-101722 from the present applicant discloses that it has been found that, in a module which includes a semiconductor laser element with an isolation wavelength of 350 to 450 nm, if the density of oxygen in the sealed atmosphere becomes excessively high, laser characteristics deteriorate accordingly.
Using a laser module for which a well-known scrubbing process has been implemented, and in which the oscillation wavelength of a semiconductor laser element employed in the laser module is switched between 410 nm, 810 nm and 980 nm, it has been evaluated how reliability varies in accordance with oxygen density in the sealed atmosphere. With the laser module using the semiconductor laser element with a wavelength of 410 nm, an effect of improvement in laser characteristics in accordance with an increase in oxygen density, with a rate of deterioration of the module over time being dependent on the oxygen density, as was exhibited by the modules that used infra-red wavelengths of 810 nm and 980 nm, was not observed.
That is, for laser light of the infra-red wavelengths 810 nm and 980 nm, a decomposition reaction of hydrocarbon compounds that are deposited on optical component surfaces on the optical path of the laser, such as a fiber incidence end surface, a lens and the like in the module, which is understood to improve reliability over time, becomes more active with an increase in oxygen density. In contrast, for laser light with a wavelength of 410 nm, reliability conversely deteriorated when oxygen density was 100 ppm or greater.
This is because, in the range of oxygen density of 100 ppm or more, deposition of silicon-based compounds at a fiber end face focus portion became remarkable. Similarly to the hydrocarbon compounds, optical absorption occurred at these deposits of silicon-based compounds. Thus, reliability over time was extremely adversely affected by continuous oscillation.
That is, the hydrocarbon deposits generated by reaction of the laser light with the hydrocarbon gases are decomposed to carbon dioxide (CO2) and water (H2O) in a gas atmosphere that includes oxygen in at least predetermined amounts, and thus eliminated. However, in addition to the hydrocarbons, silicon compounds are also included in the deposits. These deposits of silicon compounds cannot be decomposed or eliminated merely by including oxygen in the atmosphere. The deposited silicon compounds are generated by photochemical reactions of the laser light with organic compound gases that include silicon (Si) atoms (below referred to as organic silicon compounds), such as in siloxane combinations (Si—O—Si), silanol groups (—Si—OH) and the like. Moreover, the presence of oxygen in the atmosphere accelerates the reaction rates of these photochemical reactions.
The silicon compounds referred to herein include inorganic silicon oxides (SiOx), organic silicon compounds, silicon carbide compounds, organic silicon carbide compounds and the like, which are compounds having structures which, whether organic or inorganic, respectively include silicon atoms. Further, the organic silicon compound gases are gases which are generated from silicone-related materials which are employed at arbitrary locations during a module production process. If these gases adhere to surfaces of respective components in the module, small amounts of the organic silicon compound gases are included in the sealed atmosphere when the module is sealed and employed.
The gas components that are present during the production process cannot be completely eliminated simply by equipping a usual clean room with a sealed gas purification device. In order to eliminate this gas, large investments in equipment are necessary. Moreover, even if well-known degreasing and scrubbing processes are implemented, it is impossible to prevent organic silicon compound gas from being mixed in with the atmosphere of fabrication processing.
Thus, as described above, even in a case in which oxygen is included in a sealed atmosphere in order to prevent deposition of hydrocarbon compounds, if the oxygen content is too great, then deposition of silicon compounds will increase, laser characteristics will deteriorate, and reliability will become poor. Moreover, a fiber incidence end face and optical components such as lenses and the like inside the module are fixed with adhesive, wax or the like, and it is not possible to replace these.